Due to increasingly stringent environmental regulations, a great deal of pressure is levied upon automotive manufacturers to reduce the level of vehicle exhaust emissions, for example, hydrocarbons, nitrogen oxides (NOx) and carbon monoxide. As is well known, an effective method of reducing exhaust emissions is to supply fuel to the combustion space at high injection pressures (around 2000 bar for example) and to adopt nozzle outlets of a small diameter in order to optimise the atomisation of fuel and so improve efficiency and reduce the levels of hydrocarbons in the exhaust gases. Although the above approach is effective at improving fuel efficiency and reducing harmful engine exhaust emissions, an associated drawback is that reducing nozzle outlet diameter conflicts against the requirement for high fuel injection flow rates at high engine loads and so can compromise vehicle performance.
So-called “variable orifice nozzles” (VONs) enable variation in the number of orifices (and therefore the total orifice area) used to inject fuel into the combustion space at different engine loads. Typically, such an injection nozzle has at least two sets of nozzle outlets with first and second valves being operable to control whether fuel injection occurs through only one of the sets of outlets or through both sets simultaneously. In a known injection nozzle of this type, as described in the Applicant's co-pending European patent application no. EP04250928, the fuel flow to a first (upper) set of nozzle outlets is controlled by an outer valve and the fuel flow to a second (lower) set of nozzle outlets is controlled by an inner valve. The inner valve is lifted by the outer valve only after the flow of fuel through the first set of nozzle outlets has reached a sufficient rate. An injection nozzle of this type enables selection of a small total nozzle outlet area in order to optimise engine emissions at relatively low engine loads. On the other hand, a large total nozzle outlet area may be selected so as to increase the total fuel flow at relatively high engine loads.
Although beneficial in many ways, such nozzles do have associated problems. For instance, if the valves do not lift with perfect concentricity, high side loads can be generated due to the hydraulic pressure being significantly lower on the side of the outer valve closest to the nozzle body. Under some conditions these side loads can be high enough to prevent the outer valve closing.
One aspect of the present invention relates to a variable orifice nozzle which aims to have the advantages of the above designs, but which serves to alleviate or overcome the aforementioned side load problem.